arc_default_max on Linux should match FreeBSD

Commits 518b487 and 23bdb07 changed the default ARC size limit on Linux systems to 1/2 of physical memory, which has become too strict for modern systems with large amounts of RAM. This patch changes the default limit to match that of FreeBSD, so ZFS may have a unified value on both platforms. Reviewed-by: George Melikov <[email protected]> Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Edmund Nadolski <[email protected]> Closes openzfs#15437
truenas · Nov 15, 2023 · 70b81b0 · 70b81b0
1 parent f0eca96
commit 70b81b0
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diff --git a/man/man4/zfs.4 b/man/man4/zfs.4
@@ -638,10 +638,7 @@ Max size of ARC in bytes.
 If
 .Sy 0 ,
 then the max size of ARC is determined by the amount of system memory installed.
-Under Linux, half of system memory will be used as the limit.
-Under
-.Fx ,
-the larger of
+The larger of
 .Sy all_system_memory No \- Sy 1 GiB
 and
 .Sy 5/8 No \(mu Sy all_system_memory

diff --git a/module/os/linux/zfs/arc_os.c b/module/os/linux/zfs/arc_os.c
@@ -80,12 +80,18 @@ static struct notifier_block arc_hotplug_callback_mem_nb;
 
 /*
  * Return a default max arc size based on the amount of physical memory.
+ * This may be overridden by tuning the zfs_arc_max module parameter.
  */
 uint64_t
 arc_default_max(uint64_t min, uint64_t allmem)
 {
-	/* Default to 1/2 of all memory. */
-	return (MAX(allmem / 2, min));
+	uint64_t size;
+
+	if (allmem >= 1 << 30)
+		size = allmem - (1 << 30);
+	else
+		size = min;
+	return (MAX(allmem * 5 / 8, size));
 }
 
 #ifdef _KERNEL